<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        *{
            margin: 0;
            padding: 0;
        }
        .left{
            width: 200px;
            height: 200px;
            background: #000;
            float: left;
        }
        .right{
            width: 100%;
            height: 200px;
            background: red;
            margin-left: 200px;
            top:0;
            float: right;
        }
    </style>
</head>
<body>
<div class="left"></div>
<div class="right"></div>
</body>
</html>